CBSE Important Questions Class 8 Science Chapter 4 – Electricity: Magnetic and Heating Effects

Class 8 Science Chapter 4 Electricity: Magnetic and Heating Effects is part of the new NCERT Curiosity textbook. It explains what happens when an electric current flows through a wire. Students learn the symbols used in circuit diagrams, the heating effect of current and the devices based on it such as the electric bulb, iron and fuse, and the magnetic effect of current, which gives us the electromagnet, the electric bell and many machines.

These CBSE important questions for Class 8 Science Chapter 4 cover the symbol of an electric cell, the clock rule for finding the poles of an electromagnet, the deflection of a compass near a circuit, the parts of an electric bell, what an electromagnet is made of, the electric fuse as a safety device, and why the filament of a bulb glows. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. Draw the symbol for an electric cell and label its positive and negative terminals.

Answer: The symbol of an electric cell is drawn as two parallel lines of different lengths.
• The longer, thinner line stands for the positive terminal (+).
• The shorter, thicker line stands for the negative terminal (−).
When two or more such cells are joined end to end, the arrangement is called a battery.

Q2. How does the Clock Rule help determine the polarity of an electromagnet?

Answer: The Clock Rule tells us which end of a coil becomes the north pole and which becomes the south pole, by looking at the direction in which the current flows in the coil.
• If the current flows clockwise when seen from one end, that end behaves as the south pole.
• If the current flows anticlockwise, that end behaves as the north pole.
This rule is useful in motors, generators and other electrical machines, where the magnetic poles must be lined up correctly for the device to work.

Q3. A compass needle kept near an electric circuit shows
(a) no effect
(b) deflection
(c) reflection
(d) stillness

Answer: (b) deflection
A current flowing in a wire produces a magnetic field around it. The compass needle is a small magnet, so it turns and lines up with this field. This deflection is what first showed scientists that electricity and magnetism are linked.

Q4. An electric bell
(a) has a filament
(b) has an electromagnet
(c) works on the heating effect of current
(d) can be used as a switch

Answer: (b) has an electromagnet
An electric bell works on the magnetic effect of current. The coil inside becomes an electromagnet, attracts the iron strip, and the hammer strikes the gong to make the sound. A filament belongs to a bulb, and the heating effect is used in an iron or a heater, not in a bell.

Q5. An electric bell mainly consists of
(a) a coil of wire, an iron piece, a hammer, an iron strip and a contact screw
(b) a coil of wire, a magnet and a contact screw
(c) a magnet, a contact screw and a hammer
(d) an iron piece, a contact screw and a hammer

Answer: (a) a coil of wire, an iron piece, a hammer, an iron strip and a contact screw
All five parts are needed. The coil wound on the iron piece forms the electromagnet, which pulls the iron strip. The hammer fixed to the strip hits the gong, and as the strip moves it breaks contact at the screw, so the current stops and the strip springs back. This happens again and again, making the bell ring continuously.

Q6. An electromagnet consists of a core of ___________ wrapped around with a coil of ________ copper wire.
(a) Soft iron, insulated
(b) Aluminium, insulated
(c) Soft iron, uncovered
(d) Magnet, uncovered

Answer: (a) Soft iron, insulated
Soft iron is used for the core because it becomes a strong magnet when the current flows and loses its magnetism as soon as the current is switched off. The copper wire must be insulated so that the current goes round and round the coil instead of short-circuiting between the turns.

Q7. Name the safety device which is based on the heating effect of an electric current.

Answer: The electric fuse is the safety device based on the heating effect of an electric current.
A fuse has a thin wire of a metal with a low melting point. If too much current flows, the wire heats up and melts, which breaks the circuit and protects the appliances and the wiring from damage or fire.

Q8. What happens to the electric wire when electric current passes through it?

Answer: The wire gets heated up. This is called the heating effect of electric current. As the current flows, it has to overcome the resistance of the wire, and this produces heat. The thinner the wire and the larger the current, the more heat is produced.

Q9. The filament of an electric bulb glows on passage of current
(a) due to the heating effect of an electric current
(b) because the filament is very thin
(c) because a chemical reaction takes place inside the bulb
(d) due to the magnetic effect of an electric current

Answer: (a) due to the heating effect of an electric current
The filament is made of tungsten, which has a very high melting point and high resistance. When current passes through it, the filament becomes white hot and gives out light. No chemical reaction and no magnetic effect is involved.

Q10. An electric iron
(a) works on the principle of magnetic effect of an electric current
(b) can act as an electromagnet
(c) works on the principle of heating effect of an electric current
(d) is a source of light

Answer: (c) works on the principle of heating effect of an electric current
An electric iron has a coil of high-resistance wire inside. When the current flows, this coil becomes hot, and the heat passes to the metal base, which is then used to press clothes.

Q11. The symbol for an electric cell is
(a) two parallel lines of the same thickness
(b) two parallel lines of the same size
(c) two lines cutting each other
(d) a longer line followed by a shorter and thicker line

Answer: (d) a longer line followed by a shorter and thicker line
The two lines must be different, otherwise we could not tell the terminals apart. The longer thin line is the positive terminal and the shorter thick line is the negative terminal.

Q12. What is an electromagnet?

Answer: An electromagnet is a temporary magnet made by passing an electric current through a coil of insulated wire wound around a piece of soft iron.
While the current flows, the coil and the iron core behave like a magnet. As soon as the current is switched off, the magnetism disappears. This is its great advantage: it can be switched on and off.
Uses: strong electromagnets in cranes lift very heavy iron objects and separate magnetic material from junk in scrapyards; small electromagnets are used by doctors to take out tiny iron pieces that have accidentally gone into a patient's eye.

Related Study Material on Extramarks

• CBSE Important Questions Class 8 Science Chapter 1
• CBSE Important Questions Class 8 Science Chapter 2
• CBSE Important Questions Class 8 Science Chapter 3
• CBSE Important Questions Class 8 Science Chapter 5
• CBSE Important Questions Class 8 Science Chapter 6
• CBSE Important Questions Class 8 Science Chapter 7
• CBSE Important Questions Class 8 Science Chapter 8
• CBSE Important Questions Class 8 Science Chapter 9
• CBSE Important Questions Class 8 Science Chapter 10
• CBSE Important Questions Class 8 Science Chapter 11
• CBSE Important Questions Class 8 Science Chapter 12
• CBSE Important Questions Class 8 Science Chapter 13